6SL7 diff pair - how to set up???? - Page 2 - diyAudio
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Old 12th March 2005, 09:03 PM   #11
SY is offline SY  United States
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That's an interesting idea. Haven't tried it but I don't see why it wouldn't work, assuming the 6SL7 is happy in that kind of service (it's not a great tube for cascode).
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Old 12th March 2005, 09:31 PM   #12
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Andy,

I run 1.6mA and 100K in a Sylvania 6SL7 I use in one of my SE hybrid amps. B+ is critical to sonics; I use 340V, with 175V on the plate. Vgk is 1.1 volts, and overload margin about 3Vpp. Av is 43, pretty reasonable. I've never had a moment's bother with this tube, which I consider one of the most mellifluous around, although it does widen the sound stage a little unnaturally.

You could run it straight onto the outputs, Cary does this, but my own feeling is that a cathode follower using something like a 6SN7 (very neutral tube) is a very good idea. If you use a CF, you can direct couple it by use of a negative supply, using the grid of the CF to bias the output stage. This is an elegant solution which sounds good. You should run your 6SN7 around 10mA.

Keeping tube count down is a worthy goal, but not always practical.

Hope this is helpful,

Cheers,

Hugh
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Old 12th March 2005, 11:59 PM   #13
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If you use a CF, you can direct couple it by use of a negative supply, using the grid of the CF to bias the output stage.>

Hugh - yes, that's a solution. But right now I have 47 ohm unbypassed resistors on the output cathodes and fixed bias adjustable by a pot to the grids, i.e. combination or 'split' bias. So by direct coupling I'd lose that. For only one capacitor, I'd be more inclined to direct couple the first diff pair to the second, and cap couple the second to the outputs. You could do either. My argument for having adjustable bias on the 12b4 is that they test all over the place, and it really helps to be able to adjust them - they're a bugger to match into pairs for some reason, quite the worst tube for variance I've come across.
I'm a bit intrigued by SRPP 6SL7 over a common CCS - I haven't seen an actual schematic for this, is it possible? Andy
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Old 13th March 2005, 12:19 AM   #14
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Hi,

Andy,

You're requirements seem very similar to what Bob Danielak uses to drive a PP 2A3, the 12B4 being it's smaller cousin so to speak.

Maybe his work helps you further:

BOB D's 2A3 PP

Cheers,
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Old 13th March 2005, 12:33 AM   #15
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Andy, You're requirements seem very similar to what Bob Danielak uses to drive a PP 2A3, the 12B4 being it's smaller cousin so to speak. Maybe his work helps you further: BOB D's 2A3 PP>>

That's very interesting. Look at the anode resistors on the 6SL7 - 47k! That's unusual.
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Old 13th March 2005, 12:52 AM   #16
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Hi,

Quote:
I'd like to ask, if I did try the 6SL7 in SRPP - a popular SET front end - then how do I configure it in PP?
If you want it to drive a PP stage you'd need two SRPP stages in paraphase configuration to give you both gain and phase splitting.
Alternatively you could increase gain by changing the SRPP to a mu-follower or by using a Kimmel TTSA.
Personally I'd stick with either the diff amp and CCS in the tail as it seems to sound best to my ears.

Cheers,
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Old 13th March 2005, 09:14 AM   #17
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If you want it to drive a PP stage you'd need two SRPP stages in paraphase configuration to give you both gain and phase splitting.>

That;s one idea - I guess another would be to use a transformer splitter into two equal SRPPs.
Would using a CCS in the tail of a diff SRPP upset the top tubes or not?
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Old 15th March 2005, 06:39 AM   #18
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Default srpp push pull driver

I am presently using srpp 6sn7's to drive 2a3's in a push pull amp using the paraphase arrangement suggested by Frank. I had to tweak the values of the resistors in the divider network a bit, but it works very well, drive is equal within a few tenths of a volt at full output. I also am using two se output transformers in each of these amps, connected in parallel but out of phase. Yields slightly better than 7 watts with a 2.25 volt sensitivity. To my ears, this arrangement sounds very good.
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Old 15th March 2005, 08:02 AM   #19
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Mmm - SRPP in paraphase - very interesting - would it be possible to have a look at your circuit? Andy
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Old 15th March 2005, 11:20 AM   #20
JimW is offline JimW  United States
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Default srpp paraphase

I'll see if I can come up with a scannable drawing - might take a day or so. The circuit is similar to the JEL srpp circuit, except I set it up with 6sn7 instead of 6sl7, as I had trouble with the l7's hf response. They were operating at .75 ma, and showed rolloff at 5khz. I am running the 6sn7 at 4.5 ma, and the are flat out past 25khz. The paraphase divider is 470k/58k/470k. The 58k is the value that has to be tweaked, depending on gain of the stage. I have both srpp cathodes bypassed. The divider is on the 2a3 side of the capacitor. I also used 470k grid leaks and 1k grid dtoppers on the 2a3's.
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